UNDERSTANDING VARIATION IN PARTITION COEFFICIENT, Kd ...

Percent Distribution
100
80
60
40
20
0
+
PuO2
2+
PuF2
Pu 3+
3 4 5 6 7 8 9 10
5.39
pH
2-
Pu(OH)2(CO3)2
4-
Pu(HPO4)4
o
Pu(OH)4 (aq)
Figure 5.3. Calculated distribution of plutonium aqueous species as a function of pH for the
water composition in Table 5.1. [The species distribution is based on a
concentration of 3.2 x 10 -10 mg/l (1.36 x 10 -15 M) total dissolved plutonium.]
Laboratory studies conducted by Rai et al. (1980a), Delegard (1987), and Yamaguchi et al.
(1994) indicated that a freshly precipitated amorphous PuO 2@xH 2O phase controls the equilibrium
solubility of plutonium. Solubility on aged precipitates by Rai et al. (1980a) and Delegard (1987)
also showed that equilibrium plutonium concentrations would be controlled by a partially
crystallized PuO 2@xH 2O phase at concentrations about 2 orders of magnitude less than that of
amorphous PuO 2@xH 2O. Therefore, under oxidizing conditions, amorphous PuO 2@xH 2O, if present
in soils, may control soluble plutonium concentrations near 10 -8 M. Under alkaline conditions
with high dissolved carbonate concentrations, dissolved plutonium concentrations may increase to
micromolar levels. When dissolved carbonate is not present, PuO 2@xH 2O may control plutonium
concentrations at about 10 -10 M (Rai et al., 1980a).